The invention relates to communications systems and methods, and more particularly to a system and method for video communications which implements an image compression scheme.
In video communications, images are coded in accordance with an image compression scheme to reduce the required bandwidth for transmitting the images. In accordance with such well known image compression standards as the MPEG, MPEG-2, motion-JPEG and JPEG standards, an image coding process typically includes performing a block based frequency transform, e.g., discrete cosine transform (DCT), on an image to be transmitted. The resulting DCT coefficients are quantized or mapped to different quantization steps to render an approximate representation thereof. If the available transmission bandwidth is relatively small, with respect to the complexity of the image to be transmitted, the size of the quantization steps needs to be relatively large. In that case, the resulting coarse quantization of the DCT coefficients introduces coding artifacts into the transmitted image, which include, e.g., speckle-like distortions near edges of objects appearing in a recovered image.
Attempts have been made to reduce effects of the coding artifacts by postprocessing the recovered image. Since the artifacts typically comprise high frequency components, such attempts invariably involve use of a postprocessor including a low-pass filter to filter out those components in the recovered image. However, the design of such a postprocessor is normally determined by experiment in an ad hoc manner. As a result, the quality of the postprocessed image is highly dependent upon how close the actual types of image transmitted and transmission rates used in the field are to those in the experiment. Consequently, use of the prior art postprocessor to improve the image quality is, at best, precarious as the performance achieved thereby may drastically vary from one set of field conditions to another.
Accordingly, there exists a need for a dependable technique for effectively reducing the effects of the coding artifacts in an image subject to an image coding process.
We have recognized that due to the implementation of the above image coding process, which involves quantization of block based frequency transform coefficients, e.g., DCT coefficients, concerning a transmitted image, the amplitude of each non-zero frequency component of the image, when received, cannot be smaller than a threshold value, which is a function of the quantization step size. That is, any non-zero frequency component having an amplitude smaller than the threshold value must be erroneously created, perhaps, by aliasing which is incurred due to the block structure of the transform coefficients traversing a channel, which is band limited.
Thus, in accordance with the invention, using a filter arrangement which is matched to the coding of the image involving the above quantization, the received image is transformed into a set of frequency components representative thereof. The quantization step size used in the quantization may vary from frequency component to frequency component. Each frequency component having a value below a threshold value corresponding thereto is removed from the set, where the threshold value is a function of the quantization step size associated with the frequency component. Advantageously, the resulting image formed by the remaining frequency components is superior in quality to the received image, with the effects of the coding artifacts in the received image substantially reduced. In addition, the effectiveness of the inventive technique is independent of the field conditions under which the image is communicated.